The present invention relates mainly to steel sheets for automobile, and more particularly, to steel sheets having a very high strain age hardening property, excellent in press-formability such as bending workability, stretch-flanging workability, and drawing workability, in which tensile strength increases considerably through a heat treatment after press forming, and manufacturing methods thereof. The term xe2x80x9csteel sheetsxe2x80x9d as herein used shall include hot-rolled steel sheets, cold-rolled steel sheets, and plated steel sheets.
Weight reduction of automobile bodies has become in recent years a very important issue in relation to emission control for the purpose of preserving global environments. More recently, efforts are made to achieve a higher strength of automotive steel sheets and reduce steel sheet thickness.
Because many of the body parts of automobile made of steel sheets are formed by press-working, steel sheets used are required to have an excellent press-formability. In order to achieve an excellent press-formability, it is necessary to ensure a low yield strength and a high elongation. Stretch-flanging may be frequently applied in some cases, so that it is also necessary to have a high hole-expanding ratio. In general, however, a higher strength of steel sheet leads to an increase in yield strength and deterioration of shape freezability, and tends to result in a lower elongation and a poorer hole-expanding ratio, thus leading to a lower press-formability. As a result, there as conventionally been an increasing demand for high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets and high-strength plated steel sheets having high elongation and excellent in press-formability.
Importance is now placed on safety of automobile body to protect a driver and passengers upon collision, and for this purpose, steel sheets are demanded to have an improved impact resistance as a standard of safety upon collision. For the purpose of improving impact resistance, a higher strength in a completed automobile is more favorable. There has therefore been the strongest demand for high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets and high-strength plated steel sheets having a low strength and a high elongation and excellent in press-formability upon forming automobile parts, and having a high strength and excellent in impact resistance in completed products.
To satisfy such a demand, a steel sheet high both in press-formability and strength was developed. This is a baking hardening type steel sheet of which yield stress increases by applying a baking treatment usually including holding at a high temperature of 100 to 200xc2x0 C. after press forming. This steel sheet is based on a process comprising the steps of controlling the content of C remaining finally in a solid-solution state (solute C content) within an appropriate range, keeping mildness, satisfactory shape freezability and elongation during press forming, preventing movement of dislocation introduced during press forming by the residual solute C fixed to it during the baking treatment after press forming, thereby causing an increase in yield stress. However, in this baking hardening type automotive steel sheet, while yield stress can be increased, it was impossible to increase tensile strength.
Japanese Examined Patent Application Publication No. 5-24979 discloses a baking hardening high-strength cold-rolled steel sheet having a chemical composition comprising from 0.08 to 0.20% C, from 1.5 to 3.5% Mn and the balance Fe and incidental impurities, and having a structure composed of uniform bainite containing up to 5% ferrite or bainite partially containing martensite. The cold-rolled steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979 has an object to achieve a high baking hardening amount conventionally unavailable through conversion of structure from the conventional structure mainly comprising ferrite into a structure mainly comprising bainite, by rapidly cooling the steel sheet after continuous annealing within a temperature range of from 400 to 200xc2x0 C. in the cooling step and then slowly cooling the same. In the steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979, however, while a high baking hardening amount conventionally unavailable is obtained through an increase in yield strength after baking, it is yet impossible to increase tensile strength, and there still remains a problem in that improvement of impact resistance cannot be expected.
On the other hand, several hot-rolled steel sheets are proposed with a view to increasing not only yield stress but also tensile strength by applying a heat treatment after press forming.
For example, Japanese Examined Patent Application Publication No. 8-23048 proposes a manufacturing method of a hot-rolled steel sheet, comprising the steps of reheating a steel containing from 0.02 to 0.13% C, up to 2.0% Si, from 0.6 to 2.5% Mn, up to 0.10% sol. Al, and from 0.0080 to 0.0250% N to a temperature of at least 1,100xc2x0 C., applying a hot rolling end finish rolling at a temperature of from 850 to 950xc2x0 C., then cooling the hot-rolled steel sheet at a cooling rate of at least 15xc2x0 C./second to a temperature of under 150xc2x0 C., and coiling the same, thereby achieving a composite structure mainly comprising ferrite and martensite. In the steel sheet manufactured by the technique disclosed in Japanese Examined Patent Application Publication No. 8-23048, however, while tensile strength is increased, together with yield stress, by strain age hardening, a serious problem is posed in that coiling of the steel sheet at a very low coiling temperature as under 150xc2x0 C. results in large dispersions of mechanical properties. Another problems include large dispersions of increment of yield stress after press forming and baking treatments, as well as an insufficient press-formability resulting from a low hole-expanding ratio (xcex) and a decreased stretch-flanging workability.
On the other hand, for some portions, automotive parts are required to have a high corrosion resistance. A bot-dip galvanized steel sheet is suitable as a material applied to portions required to have a high corrosion resistance, and a particular demand exists for hot-dip galvanized steel sheets excellent in press-formability during forming, and is considerably hardened by a heat treatment after forming.
To respond to such a demand, for example Japanese Patent Publication No. 2802513 proposes a manufacturing method of a hot-dip galvanized steel sheet using a hot-rolled steel sheet as a substrate. The patented method comprises the steps of hot-rolling a steel slab containing up to 0.05% C, from 0.05 to 0.5% Mn, up to 0.1% Al and from 0.8 to 2.0% Cu under conditions including a coiling temperature of up to 530xc2x0 C., reducing the steel sheet surface by heating the hot-rolled steel sheet to a temperature of up to 530xc2x0 C., and hot-dip-galvanizing the sheet, whereby a remarkable hardening is available through a heat treatment after forming. In the steel sheet manufactured by this method, however, in order to obtain a remarkable hardening from the heat treatment after forming, the heat treatment temperature must be at least 500xc2x0 C., and this has posed a problem in practice.
Japanese Unexamined Patent Application Publication No. 10-310824 proposes a manufacturing method of an alloyed hot-dip galvanized steel sheet permitting expectation of an increase in strength through a heat treatment after forming, using a hot-rolled or cold-rolled steel sheet as a substrate. This method comprises the steps of hot-rolling a steel containing from 0.01 to 0.08% C, appropriate amounts of Si, Mn, P, S, Al and N, and one or more of Cr, W and Mo in a total amount of from 0.05 to 3.0%, or cold-rolling or temper-rolling the sheet and annealing the same, applying hot-dip galvanizing the sheet, and then, conducting a heating/alloying treatment. The Publication asserts that, after forming, tensile strength is increased by heating the sheet at a temperature within a range of from 200 to 450xc2x0 C. However, the resultant steel sheet involves a problem in that, because the microstructure comprises a ferrite single phase, a ferrite+pearlite, or a ferrite+bainite structure, a high elongation and a low yield strength are unavailable, resulting in a low press-formability.
Japanese Unexamined Patent Application Publication No. 11-199975 proposes a hot-rolled steel sheet for working excellent in fatigue property, containing from 0.03 to 2.0% C, appropriate amounts of Si, Mn, P, S and Al, from 0.2 to 2.0% Cu, and from 0.0002 to 0.002% B, of which the microstructure is a composite structure having ferrite as a main phase and martensite as the second phase, and the state of presence of Cu in the ferrite phase in a solid-solution state and/or precipitation of up to 2 nm. The proposed steel sheet has an object based on a fact that fatigue limit ratio is remarkably improved only when compositely adding Cu and B, and achieving the finest state of Cu as up to 2 nm. For this purpose, it is essential to end hot finish rolling at a temperature of at least the Ar3 transformation point, air-cool the sheet within a temperature region of from Ar3 to Ar1 in cooling for a period of from 1 to 10 seconds, then cool the sheet at a cooling rate of at least 20xc2x0 C./second, and coil the cooled sheet at a temperature of up to 350xc2x0 C. A low coiling temperature of up to 350xc2x0 C. poses a problem of causing a serious deformation of the shape of the hot-rolled steel sheet, thus preventing industrially stable manufacture.
The present invention was developed in view of the fact that, in spite of the strong demand as described above, a technique for industrially stably manufacturing a steel sheet satisfying these properties has never been proposed, and has an object to favorably solve the problems described above and to provide a high-strength steel sheet suitable as an automotive steel sheet, having an excellent press-formability, and excellent in strain age hardening property causing tensile strength to increase considerably through a heat treatment at a relatively low temperature after press-forming, and a manufacturing method permitting stable production of such a high-strength steel sheet. The term xe2x80x9csteel sheetsxe2x80x9d as herein used shall include hot-rolled steel sheets, cold-rolled steel sheets and plated steel sheets.
To achieve the above-mentioned object of the invention, the present inventors carried out extensive studies on the effect of the steel sheet structure and alloying elements on strain age hardening property. As a result, the following findings were obtained. It is possible to obtain a high strain age hardening bringing about an increase in yield stress, and in addition, a remarkable increase in tensile strength, after application of a pre-strain treatment of an amount of prestrain of 5% or more and a heat treatment at a relatively low temperature within a range of from 150 to 350xc2x0 C. There is thus available a steel sheet having a satisfactory elongation, a low yield strength and a high hole expanding ratio, and excellent in press-formability.
On the basis of the novel findings as described above, the present inventors carried out further extensive studies and found that the above-mentioned phenomenon occurred in steel sheets not containing Cu as well. When a prestrain is imparted by using a steel sheet containing one or more of Mo, Cr and W in place of Cu, and achieving a ferrite+martensite composite structure, and a heat treatment was applied at a low temperature, very fine carbides were formed to strain-induced-precipitate in martensite, resulting in an increase in tensile strength. The strain-induced precipitation upon heating to a low temperature was found to become more remarkable by containing one or more of Nb, V and Ti, in addition to one or more of Mo, Cr and W.
The present invention was completed through further studies on the basis of the aforementioned findings. The gist of the invention is as follows:
(1) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising a structure having ferrite phase as a main phase forming a composite structure with a secondary phase containing martensite phase in an area ratio of 2% or more.
(2) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more as in (1) above, wherein the steel sheet is a hot-rolled steel sheet.
(3) A steel sheet according to (2) above, excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising, in weight percentage: 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, from 0.5 to 3.0% Cu and the balance Fe and incidental impurities.
(4) A steel sheet according to (3) above, containing, in weight percentage, one or more selected from the following groups A to C, in addition to the above-mentioned chemical composition:
group A: Ni: 2.0% or less;
group B: one or two of Cr and Mo: 2.0% or less in total; and
group C: one or more of Nb, Ti and V: 0.2% or less in total.
(5) A steel sheet according to (2) above, excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, having a chemical composition comprising, in weight percentage: 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, one or more selected from the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, 2.0% or less in total, and the balance Fe and incidental impurities.
(6) A steel sheet according to (5) above, excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, further comprising, in addition to the above-mentioned chemical composition, in weight percentage, one or more selected from the group consisting of Nb, Ti, and V, 2.0% or less in total.
(7) A manufacturing method of a steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising the steps, when hot-rolling a steel slab having a chemical composition comprising, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, and from 0.5 to 3.0% Cu, or additionally containing one or more selected from the following groups A to C:
group A: Ni: 2.0% or less;
group B: one or two of Cr and Mo: 2.0% or less in total; and
group C: one or more of Nb, Ti and V: 0.2% or less in total, and preferably the balance Fe and incidental impurities, into a hot-rolled steel sheet having a prescribed thickness, carrying out the hot rolling with a finish rolling end temperature FDT of the Ar3 transformation point or more, then after the completion of the finish rolling, cooling the hot-rolled steel sheet to a temperature region from the (Ar3 transformation point) to the (Ar1 transformation point) at a cooling rate of 5xc2x0 C./second or more, air-cooling or slowly cooling the sheet within the temperature region for a period of from 1 to 20 seconds, then cooling the sheet again at a cooling rate of 5xc2x0 C./second or more, and coiling the sheet at a temperature of 550xc2x0 C. or below.
(8) A manufacturing method of a hot-rolled steel sheet excellent in press-formability and in strain age hardening property as typical represented by a xcex94TS of 80 MPa or more, according to (6) above, wherein the steel slab has a chemical composition containing, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, and further containing one or more selected from the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr, and from 0.05 to 2.0% W, 2.0% or less in total, or further containing one or more selected from the group consisting of Nb, Ti and V, in an amount of 2.0% or less in total, and preferably, the balance Fe and incidental impurities.
(9) A manufacturing method of a hot-rolled steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (7) or (8) above, wherein all or part of the finish rolling comprises lubrication rolling.
(10) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (1) above, which is a cold-rolled steel sheet.
(11) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (10) above, comprising, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, from 0.5 to 3.0% Cu, and the balance Fe and incidental impurities.
(12) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (11) above, containing, in weight percentage, one or more selected from the following groups A to C, in addition to the above-mentioned chemical composition:
group A: Ni: 2.0% or less;
group B: one or two of Cr and Mo: 2.0% or less in total; and
group C: one or more of Nb, Ti and V: 0.2% or less in total.
(13) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (10) above, having a chemical composition comprising, in weight percentage, in addition to the above-mentioned chemical composition, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, one or more selected from the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, 2.0% or less in total, and the balance Fe and incidental impurities.
(14) A steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (13) above, further comprising, in addition to the above-mentioned chemical composition, in weight percentage, one or more selected from the group consisting of Nb, Ti and V, 2.0% or less in total.
(15) A manufacturing method of a cold-rolled steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising the steps of using a steel slab having a chemical composition containing, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, and from 0.5 to 3.0% Cu, or further containing one or more selected from the following groups A to C:
group A: Ni: 2.0% or less;
group B: one or two of Cr and Mo: 2.0% or less in total; and
group C: one or more of Nb, Ti and V: 0.2% or less in total, and preferably, the balance Fe and incidental impurities as a material; a hot rolling step of applying hot rolling to the material into a hot-rolled steel sheet; a cold rolling step of applying cold rolling to the hot-rolled steel sheet into a cold-rolled steel sheet; and a recrystallization annealing step of applying recrystallization annealing into a cold-rolled annealed steel sheet; these steps being sequentially applied; wherein the recrystallization annealing is conducted in a ferrite+austenite dual phase region within a temperature range of from Ac1 transformation point to Ac3 transformation point.
(16) A manufacturing method of a cold-rolled steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (15) above, wherein the steel slab has a chemical composition containing, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, and further containing one or more selected from the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr, and from 0.05 to 2.0% W, or further containing one or more of Nb, Ti and V, 2.0% or less in total, and preferably, the balance Fe and incidental impurities.
(17) A manufacturing method of a cold-rolled steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (15) or (16) above, wherein the hot rolling is conducted under conditions including a heating temperature of the material of 900xc2x0 C. or more, a finish rolling end temperature of 700xc2x0 C. or more, and a coiling temperature of 800xc2x0 C. or below.
(18) A manufacturing method of a cold-rolled steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to any one of (15) to (17) above, wherein all or part of the hot rolling comprises lubrication rolling.
(19) A hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of the hot-rolled steel sheet according to any one of (2) to (6) above.
(20) A hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of the cold-rolled steel sheet according to any one of (10) to (14) above.
(21) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, comprising the steps of using a steel sheet having a chemical composition containing, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, 0.02% or less N, and from 0.5 to 3.0% Cu, or further containing one or more selected from the following groups:
group A: 2.0% or less Ni;
group B: one or two of Cr and Mo: 2.0% or less in total; and
group C: one or more of Nb, Ti and V: 0.2% or less in total, preferably the balance Fe and incidental impurities, applying annealing comprising heating to a dual phase region of ferrite+austenite within a temperature range of from Ac3 transformation point to Ac1 transformation point to the steel sheet on a line for conducting continuous hot-dip galvanizing, and then, performing a hot-dip galvanizing treatment, thereby forming a hot-dip galvanizing layer on the surface of the steel sheet.
(22) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to (21) above, wherein the steel sheet is replaced by a steel sheet having a chemical composition containing, in weight percentage, 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, 0.1% or less P, 0.02% or less S, 0.1% or less Al, and 0.02% or less N, and further comprising one or more selected from the group consisting of from 0.05 to 2.0% Mo, from 0.05 to 2.0% Cr and from 0.05 to 2.0% W, 2.0% or less in total, or further containing one or more of Nb, Ti and V in an amount of 2.0% or less in total, preferably the balance Fe and incidental impurities.
(23) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by as xcex94TS of 80 MPa or more, according to (21) or (22) above, wherein, prior to the annealing, a preheating treatment of heating the sheet at a temperature of 700xc2x0 C. or more on a continuous annealing line, and then applying a pretreatment comprising a pickling treatment.
(24) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to any one of (21) to (23) above, comprising the steps of conducting the hot-dip galvanizing treatment to form a hot-dip galvanizing layer on the surface of the steel sheet, and then, performing an alloying treatment of the hot-dip galvanizing layer.
(25) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to any one of (21) to (24) above, wherein the steel sheet is a hot-rolled steel sheet manufactured by hot-rolling the material having the chemical composition under conditions including a heating temperature of 900xc2x0 C. or more, a finish rolling end temperature of 700xc2x0 C. or more and a coiling temperature of 800xc2x0 C. or below, or a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet.
(26) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, further comprising a step of applying a hot-dip galvanizing treatment to the hot-rolled steel sheet resulting from the manufacturing method of a hot-rolled steel sheet according to any one of (7) to (9) above to form a hot-dip galvanizing layer on the surface of the hot-rolled steel sheet.
(27) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, further comprising a step of applying a hot-dip galvanizing treatment to the cold-rolled steel sheet resulting from the manufacturing method of a cold-rolled steel sheet according to any one of (15) to (18) above to form a hot-dip galvanizing layer on the surface of the cold-rolled steel sheet.
(28) A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability and in strain age hardening property as typically represented by a xcex94TS of 80 MPa or more, according to any one of (26) and (27) above, further comprising the step of carrying out an alloying treatment after the hot-dip galvanizing treatment.